It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) forbade the castration of horses and other quadrupeds, and Sayyiduna Ibn Umar (may Allah be pleased with them both) says that their physical development lies in this.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4769
Hadith Gradingحکم دارالسلام:إسناده ضعيف لضعف عبد الله بن نافع مولي ابن عمر، وقد روي مرفوعا و موقوفا، وموقوفه هو الصحيح